There are now many different picture width-to-height ratios (aspect-ratios) in use in the field of television and video, and it is frequently necessary for images to be presented on displays having a different ratio from the one for which they were originally intended. This problem is solved by the well-known techniques of aspect-ratio conversion. It is generally highly undesirable for aspect-ratio conversion to change the relative horizontal and vertical proportions of portrayed objects, and so “borders” are often added to television pictures so as to fill parts of the raster which are necessary to fill the display area, but which contain no image content. Examples of this are the well-known “black bands” above and below images shown on displays which are not wide enough; and, “side-curtains” on displays which are not high enough. Borders may also be introduced unintentionally due to incorrect use of aspect-ratio converters, or due to incorrect definition of the active picture area in other video processing operations.
Automatic detection of borders can assist many video and image processing tasks. For example: a process may operate differently in the border areas; or, unnecessary borders, which waste usable picture area, may be removed automatically.
Borders are usually black—which ought to make them easy to detect. However, in practice, the darkest part of the picture may be darker or lighter than the border; and there may be noise present, possibly with different characteristics in the border region and the picture. Thus simple border detection algorithms that identify a defined border colour or luminance value frequently fail to provide acceptable performance.